With Canon lenses mounted on a Canon camera, I've watched it focus on something and lock, then I press the focus button again, it de-focuses and refocuses again. Why can't it just "know" that it has acquired focus and not move the second time? That part of the focusing algorithm is necessary because your camera has no knowledge that it hasn't moved or whatever you are shooting hasn't moved. It may have GPS in some models, but I've yet to hear of one with gyroscopes, accelerometers, and radar.
Edit: I see Lightmaster was thinking the same thing I was. Great minds think alike!
You know I think I could come up with any bogus behaviour by a Canon product and one or more people would find a way to justify it.
But then if a 3rd party behaves strangely, well, it is all the fault of the 3rd party and should never happen!
The point I was making above is that if you've already focused the lens on X and press the button again, it is doubtful that the lens will end up in exactly the same position as before (and by exact, I mean exact, not some "within half a millimeter.")